1. Field of the Invention
The present invention relates to data processing systems and snore particularly to systems for creating application programs or operating system programs. Still more particularly, the present invention relates to object oriented systems having tailorable persistent object storage functions.
2. Background and Related Art
The development of application and system software for data processing systems has traditionally been a time consuming task. The field of software engineering has attempted to overcome the limitations of traditional techniques by proposing new, more efficient software development models. Object oriented programming has emerged as a promising technology that will allow rapid development, implementation and customization of new software systems.
Object oriented programming uses a toolkit of system objects that can be assembled to perform the final task.
Object oriented programming uses a toolkit of system objects that can be assembled to perform the final task. Each object has certain data attributes and processes or methods that operate on that data. Data is said to be "encapsulated" by an object and can only be modified by the object methods. Methods are invoked by sending a message to an object identifying the method and supplying any needed arguments.
Object oriented systems have two important properties in addition to encapsulation. "Inheritance" is the ability to derive a new object from an existing object and inherit all properties, including methods and data structure, from the existing object. The new object may have certain unique features which are supplied as overrides or modifications to the existing class. I.e. a new subclass needs to specify only the functions and data that distinguish that class from the existing more general class.
The ability to override an existing method description is termed polymorphism because a single message to an object can be processed in different ways depending on the object itself.
Inheritance and polymorphism create a powerful structure for implementing new software systems. The software developer does not have to develop each piece of a system, he or she need only specify the unique features of the system.
The power of object oriented systems is realized through the development of system "frameworks." A framework is a collection of base classes that can be used by a system implementor to create a final systems product. The framework is defined and developed to work together as a system. Conceptually, the framework is much like a set of standard hardware components used by computer hardware builders. Each of the components has certain defined functions and interfaces and the engineer assembles these components according to a particular design to create a unique hardware system.
Objects created by a data processing system are typically maintained in the volatile memory of those systems. This allows faster processing, but does not provide a means to store data. Object oriented systems solve this problem by implementing "persistent objects." "Persistence" means that an object's state can be preserved beyond the termination of the process that created that object. The persistent framework includes methods that allow persistent objects to be stored in and retrieved from a non-volatile medium such as a hard disk drive or writable optical disk. Objects can be stored individually or grouped with other objects.
The use of Persistent Streams to capture and retain database queries and results is discussed in U.S. Pat. No. 5,161,225 to Abraham et al., commonly assigned.
Existing persistent object systems are limited by their fixed implementation of object storage. A particular object oriented product typically provides one mechanism for storing persistent objects. This mechanism includes a definition of the non-volatile medium used, the structure of object groups stored on the medium and the structure of each particular object stored on the medium.
Fixed implementations of persistent objects limit the developer flexibility to change persistent object storage to meet particular system requirements. The developer may want to change to improve individual object storage efficiency by compression, or to enhance security through encryption. Object group structure may need to be changed to implement new technologies such as object oriented databases or to allow interchange of objects between systems having different persistent object group structures.
The existing technical problem addressed by the present invention is to provide a persistent object framework that allows developer modification of group skeletal formats used for object group storage.